As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for batteries has also sharply increased as an energy source for the mobile devices. Accordingly, much research on batteries satisfying various needs has been carried out.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries, which are thin enough to be applied to products, such as mobile phones, is very high. In terms of the material for batteries, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, having high energy density, high discharge voltage, and high output stability, is very high.
Furthermore, secondary batteries may be classified based on the construction of an electrode assembly having a cathode/separator/anode structure. For example, the electrode assembly may be constructed in a jelly-roll (winding) type structure in which long-sheet type cathodes and anodes are wound while separators are disposed respectively between the cathodes and the anodes, a stacking type structure in which pluralities of cathodes and anodes having a predetermined size are successively stacked one on another while separators are disposed respectively between the cathodes and the anodes, or a stacking/folding type structure in which pluralities of cathodes and anodes having a predetermined size are successively stacked one on another, while separators are disposed respectively between the cathodes and the anodes, to constitute a bi-cell or a full-cell, and then the bi-cell or the full-cell is wound. The details of the stacking/folding type electrode assembly are disclosed in Korean Patent Application Publication No. 2001-0082058, No. 2001-0082059, and No. 2001-0082060, which have been filed in the name of the applicant of the present patent application.
However, the conventional electrode assemblies have several problems.
First, the jelly-roll type electrode assembly is manufactured by densely winding long-sheet type cathodes and anodes with the result that the jelly-roll type electrode assembly is circular or elliptical in section. Consequently, stress, generated by the expansion and contraction of the electrodes during the charge and discharge of the electrode assembly, accumulates in the electrode assembly, and, when the stress accumulation exceeds a specific limit, the electrode assembly may be deformed. The deformation of the electrode assembly results in the nonuniform gap between the electrodes. As a result, the performance of the battery is abruptly deteriorated, and the safety of the battery is not secured due to an internal short circuit of the battery. Furthermore, it is difficult to rapidly wind the long-sheet type cathodes and anodes while maintaining uniformly the gap between the cathodes and anodes, and therefore, the productivity is lowered.
Secondly, the stacking type electrode assembly is manufactured by sequentially stacking pluralities of unit cathodes and anodes. As a result, it is additionally necessary to provide a process for transferring electrode plates, which are used to manufacture the unit cathodes and anodes. Furthermore, a great deal of time and effort is required to perform the sequential stacking process, and therefore, the productivity is lowered.
Thirdly, the stacking/folding type electrode assembly considerably makes up for the defects of the jelly-roll type electrode assembly and the stacking type electrode assembly. However, a stacking process is necessary to manufacture the bi-cell or the full-cell. Consequently, the stacking/folding type electrode assembly is not a complete solution.
In conclusion, the jelly-roll type electrode assembly is preferred in the aspect of productivity, and the stacking type electrode assembly and the stacking/folding type electrode assembly are preferred in the aspect of operational performance and safety of the battery. Nevertheless, there is high necessity for a new electrode assembly that is capable of providing higher productivity and operational performance of a battery while making up for the defects of the conventional electrode assemblies.
Especially, a large-sized battery module, which is used for middle- or large-sized devices, such as electric vehicles or hybrid electric vehicles, which have lately attracted much attention, needs a large number of battery cells (unit cells). Furthermore, it is required that the large-sized battery module have a long service life characteristic. Consequently, a new electrode assembly that can solve all the above-mentioned problems is seriously needed.